Kenneth Heafield Ivan Pouzyrevsky Jonathan H. Clark Philipp Koehn Scalable Modified Kneser-Ney Language Model Estimation. 690-696 2013 ACL (2) https://aclanthology.org/P13-2121/ conf/acl/2013-2 db/conf/acl/acl2013-2.html#HeafieldPCK13